The family of Clayton Lockett, the death row inmate in Oklahoma who suffered a long and apparently traumatic execution in April, is suing a family doctor who they allege actively participated in the botched lethal injection process that killed him.

The legal complaint , lodged with the federal court for the western district of Oklahoma on Tuesday, names Dr Johnny Zellmer as a defendant both in his individual and official capacity. The lawsuit accuses him of engaging “in human medical experimentation in torturing Clayton Lockett to death”, and says that his participation in the execution was against international protocols established at the post-World War Two Nuremberg trials of Nazi doctors.

The naming of Dr Zellmer under court privilege is a rare instance of the identity of a physician who allegedly participated in an execution coming to light. Death penalty states, including Oklahoma, go to great lengths to guard the secrecy of their execution teams.

The position of doctors is particularly sensitive as physicians take the Hippocratic Oath to show “utmost respect for human life”. Where doctors have been present in the death chamber, their role has in most cases been tightly limited to assessing whether the prisoner is unconscious and then officially pronouncing death.

However, in the case of Clayton Lockett, the state has admitted that a physician was present who actively took part in killing the prisoner. The report of the internal investigation into the Lockett execution reveals that the physician stepped in to finish the job after the paramedic who had initiated the execution failed to place the IV into Lockett’s veins.

“The IV access was completed by a physician licensed as a medical doctor,” the report said.

The direct participation of the physician in helping guide lethal drugs into Lockett’s body is an apparent violation of the Hippocratic Oath. It is also a breach of the voluntary code laid out by the American Medical Association that states that doctors should not play any role that contributes to the cause of death in a legally-authorized execution.

David Lane, a civil rights lawyer in Denver who is acting for the Lockett family, said he called Zellmer a month ago and gave the physician the chance to deny that he took part in the Lockett killing. According to Lane, Zellmer replied: “Y’all have to talk to the prison about that,” and put the phone down.

The Guardian attempted to reach Zellmer but was not immediately successful.

Lockett endured a 43-minute execution involving an experimental concoction of lethal drugs in which he was observed writhing and groaning on the gurney. The investigation report indicated that there had been a shortage of appropriate needles that day, and that the physician and paramedic had failed to place the IV into the prisoner’s vein, leading to the injection of a mass of lethal drugs into his muscle.

Lane said that he had learned the identity of the doctor from an “inside source”. Zellmer is listed by the Oklahoma medical board as a fully licensed and active doctor, specializing in family and emergency medicine and practicing out of McAlester, where the state penitentiary that houses the death chamber is located.

The investigation report into Lockett’s death notes that the physician had been involved in one other execution about four or five years earlier. He had been contacted just two days before Lockett was scheduled to die as another physician had pulled out due to a scheduling conflict.

The doctor was specifically told that his duties would only involve assessing whether Lockett was unconscious and pronouncing death. It remains unclear why the doctor agreed to go further, and actively attempt to place the IV.

The report does not identify the physician, but does say that he had a license that expired on 1 July every year. Zellmer’s current license expires on 1 July 2015.

Oklahoma has introduced a law that guards the confidentiality of the execution team. Lane said that he was aware of the law, but said he had a first amendment right on a matter of supreme public concern.

“I know that it was Dr Zellmer who participated in this execution, and to deny me the right to sue the doctor who killed Clayton Lockett is to deny his family their civil rights,” he said.

The lawsuit against Zellmer also names as defendants the governor of Oklahoma, Mary Fallin, the director of the department of corrections Robert Patton and various unidentified members of the execution team. It alleges that Zellmer received payment for his services, adding that his “participation in the failed medical experiment directly caused the tortured death of Clayton Lockett”.